It has been proposed by Cavalier-Smith that genome and cell size is controlled by the influence of r- and K-selection. This hypothesis, though explaining many regularities is unable to account for the enormous cells of some vertebrates, namely dipnoan fishes and caudate amphibians. The introduction of the concept of wasteful and frugal strategies unravels this problem. Wasteful strategy consists of evolution toward high metabolism, which is a prerequisite for high mobility and a large and complicated nervous system. Frugal strategy leads to economies of energy and material but constrains several evolutionary tendencies. It is much less common than the wasteful one and is especially prominent in fishes and amphibians living in tropical and temperate fresh waters where the respiratory conditions may force these animals to reduce their metabolism to a minimal level, or even to enter a period of aestivation. Since the increase in cell size diminishes the overall energy cost for the maintenance of ionic gradients between the cell interior and the intercellular solutions, it was favoured in dipnoans and in such caudate amphibians as inhabit fresh waters where an oxygen deficiency sometimes develops. The selection forces working for the diminution of genome and cell size may be very powerful and may change these cell characters in a comparatively short time, whereas small cells and genomes may be tolerated even in environments favouring a cell increase.